1. Field of the Invention
The present invention relates to a port selection circuit for a high frequency signal, such as a transmission/reception switching circuit, which can be incorporated into an integrated circuit by utilizing a strip line technique and can also be applied to a time division multiplex apparatus and a frequency multiplex apparatus. Disclosure Document No. 357467 regarding the present invention has been accepted by the U.S. Patent and Trademark Office.
2. Description of the Prior Art
As an example of a port selection circuit, there is known a transmission/reception switching circuit which selects one of a transmitting port connected to the output of a transmitting section and a receiving port connected to the input of a receiving section, thereby connecting the selected port to an antenna port. For instance, currently used digital mobile telecommunication terminals employ different frequencies for transmission and reception signals (for example, digital cellular) or the same frequency for transmission and reception signals (for example, digital cordless phone). In any digital mobile telecommunication terminal mentioned above, a transmission/reception switching circuit is constituted by combining a transmission/reception switch (or duplexer), a transmission bandpass filter and a reception bandpass filter.
FIG. 1 is a block diagram schematically illustrating the constitution of a high frequency circuit of a conventional digital cellular. As shown in this figure, the high frequency circuit of the digital cellular comprises a transmitting section TX, a receiving section RX, a transmission/reception switch (or duplexer) TRS and an antenna port AP connected to an antenna. The transmitting section TX comprises a modulator 100 for producing a signal of a predetermined transmitting frequency by modulating an IQ signal supplied, a power amplifier 102 for power-amplifying the modulated IQ signal and a transmitting bandpass filter 104 for extracting a signal of a predetermined transmitting frequency band from the power-amplified signal. Meanwhile, the receiving section RX comprises a receiving bandpass filter 106 for extracting a signal of a predetermined receiving frequency band from a received signal supplied from the transmission/reception switch TRS, a pre-amplifier 108 for amplifying the output of the bandpass filter 106 and a demodulator 110 for demodulating the amplified received signal to an IQ signal.
In the case of a digital cordless phone, since a transmitting frequency is the same as a receiving frequency, it is possible to incorporate or integrate a transmission/reception switch, a transmitting bandpass filter and a receiving bandpass filter. However, in the case of a digital cellular, since a transmitting frequency is different from a receiving frequency, it is difficult to incorporate or integrate a transmission/reception switch, a transmitting bandpass filter and a receiving bandpass filter. Therefore, such a digital cellular must be constituted with discrete parts, resulting in such a disadvantage that the high frequency circuit thereof occupies a large physical area.